embryo- 3rd wk;3rd-8th wk devt

GASTRULATION

most characteristic event occurring the 3rd week

Begins with the formation of primitive streak (PS) on the surface

of the epiblast: cephalic end, primitive node, surrounding

primitive pit

 Cells migrate inwards towards the PS this is called Invagination,

controlled by Fibroblast Growth Factor 8 (FGF8)

FGF8 then Controls cell specification into the mesoderm by

regulating BRACHYURY (T) expression.

 Cells invaginated and displace hypoblast creat endoderm

 Between epiblast and endoderm (that was produced previously)

form mesoderm

 Cells remaining in epiblast form ectoderm. The epiblast cells

continous to move laterally, cranially and caudally. Those

epiblast cells that moves cranially are formed into two:

 Precordial plate which will be the induction of forebrain

 Oropharyngeal membrane, the future opening of oral cavity

Epiblast gives rise to all 3 germ layers in the embryo, ectoderm,

mesoderm, and endoderm (deepest). All these layers form all of

the tissues and organs during organogenesis

 The Primary villi, small capillaries, villous capillaries and the

connecting stalk (chorionic plate) supply the embryo with

nutrients and oxygen

Remember that the tropoblast will be the future placenta and

this placenta will produce oxygen and nutrients to the embryo.

In the cephalic direction, they pass on each side of the

prechordal plate. Later, the prechordal píate will be important

for induction of the forebrain

FORMATION OF NOTOCHORD

 Prenotochordal cells invaginate in primitive node and the

epiblast cells are invaginating moving lateral wise, they are at

the cranial region, are your prechordal plate. As they continue

to move lateral wise upon reaching the middle, they will form two

layers. It will be the notochondral plate. As the two layers reach

the central region, it will now becomes a single layer. It will form

the notochord because the endoderm will now be detached. So

it now only form a single layer and that will become the definitive

chord, future neural tube and future axial skeleton

- The notochord and prenotochordal cells extend cranially to

the prechordal plate (an area just caudal to the

oropharyngeal membrane) and caudally to the primitive pit.

At the point where the pit forms an indentation in the

epiblast, the neuromeric canal temporarily connects the

amniotic and yolk sac cavities.

- The cloacal membrane is formed at the caudal end of the

embryonic disc. When the cloacal membrane appears, the

posterior wall of the yolk sac forms a small diverticulum that

extends in the connecting stalk. This diverticulum, the

allantoenteric diverticulum, or allantois, appears around

the 16th day of development.

ESTABLISHMENT OF BODY AXES

 Anterior- posterior AP cranio caudal, dorso-ventral DV, left-right

LR occurs early in embryogenesis, initiated during morula stage

 AP axis forms the anterior visceral endoderm (AVE) at the

cranial end of the endoderm which will become the head region

 Mesoderm will ventralize to contribute to kidneys in the

presence of FGF8, bone morphogenetic protein 4 (BMP4),

transforming growth factor (TGF B)

 Node is the organizer

 Nodal in involved in intiating and maintaining the primitive streak

 Laterally (L-R sidedness) – primitive streak appears, FGF8

secreted, nodal expression restricted to L side. Abnormal

expression includes laterality defects: situs inversus, dextro

cardia

Lateral defects – if the nodal expressions are abnormally

expressed

Dextro cardia – if the organ that should be on the left but is

located on the right, eg. heart

Situs inversus - if the organ is positioned in a reverse manner

such as in a mirror image arrangement. For example, the organ

that is supposedly on the right but is placed on the left and one

that is placed on the left is placed on the right.

FATE ESTABLISHED DURING GASTRULATION

- Paraxial Mesoderm are those migrating at the lateral

edges of the node and from the cranial end of the streak

- Intermediate Mesoderm are cells migrating through the

mid streak region

- Lateral Plate Mesoderm forms those migrating through the

more caudal part of the streak.

- Cells migrating through the caudal most part of the streak

contribute to extraembryonic mesoderm

GROWTH OF EMBRYONIC DISC

- initially flat and almost round, gradually becomes

elongated, with a broad cephalic and a narrow caudal end.

Growth and elongation of the cephalic part of the disc are

caused by a continuous migration of cells from the primitive

streak region in a cephalic direction. At that stage, the

primitive streak shows regressive changes, rap- idly

shrinks, and soon disappears.

- That the primitive streak at the caudal end of the disc

continues to supply new cells until the end of the fourth

week has an important bearing on development of the

embryo.

- Thus, gastrulation, or formation of the germ layers,

continues in caudal segments while cranial structures are

differentiating, causing the embryo to develop

cephalocaudally.

I.PERIOD OF ORGANOGENESIS

 formation of tissues and organs

 3 germs layers, ectoderm, mesoderm, and endoderm, gives rise

to a number of specific tissues and organs

 Ectoderm germ layer (everything that is in contact with the

outside world) e.g. CNS, PNS, sensory epithelium of era, nose,

eye, skin, hair, nails, pituitary, mammary, and sweat glands

 Mesoderm germ layer (tissues that support the body) e.g

somitomeres, myotome (muscle tissue), sclerotome (cartilage,

bone), dermatome (skin)

 Endoderm germ layer (organs withing the body) e.g. GIT, RT,

UB, thyroid, parathyroid, liver, pancreas, tympanic cavity,

auditory tube

 Induction of the neural plate caused by the upregulation of

Fibroblast Growth factor (FGF), inhibition of the activities of

Bone Morphogenetic Protein 4 (BMP4), Transforming growth

factor B (TGF B)

 Formation of neural tube from neural plate (NEURULATION)

occurs by lengthening of neural plate (cranial to caudal) and

body axis, lateral edges elevate to form NEURAL FOLDS and

the depressed mid region form NEURAL GROOVE

 Neural fold approach each other in midline where they fuse and

form the cervical region that proceeds cranially and caudally.

Neural tube is formed

 Anterior cranial neuropore closes at day 25

 Posterior caudal neuropore closes at day 28

 Neurulation complete with its narrow caudal portion/ spinal cord

and broader cephalic portion/ brain vesicles

- MOLECULAR REGULATION OF NEURAL INDUCTION

- Neurulation is the process whereby the neural píate forms

the neural tube.

- NEURAL CREST. As the neural folds elevate and fuse,

cells at the lateral border or crest of the neuroectoderm

begin to dissociate from their neighbors.

The ectodermal germ layer gives rise to organs and structures that

maintain contact with the outside world:

- The central nervous system

- The peripheral nervous system

- The sensory epithelium of the ear, nose, and eye

- The epidermis, including the hair and nails

- The subcutaneous glands

- The mammary glands

- The pituitary gland

- Enamel of the teeth